The present invention relates to a fuel evaporator installed in an air-fuel passage of an internal combustion engine, which is provided with a heating element made of ceramic material having positive temperature coefficient of resistance.
Ceramic material such as barium titanate (hereinafter will be called PTC ceramic) has such characteristics that when electrified, it generates heat and when the temperature thereof reaches a specific value (called Curie temperature), the electric resistance thereof increases rapidly. Under the Curie temperature, the PTC ceramic receives a large amount of electric current to reach the Curie temperature rapidly.
Over the Curie temperature, the electric resistance becomes remarkably large and the PTC ceramic receives only a small amount of electric current, so that the PTC ceramic is not overheated. Thus, the PTC ceramic adjusts the temperature thereof by itself into about the Curie temperature.
Therefore, by using the PTC ceramic as the heating element for promoting the evaporation of the fuel supplied into the internal combustion engine, the fuel can be heated more rapidly at a cold starting time compared with the other heating means such as exhaust gas and engine coolant.
One example of the fuel evaporator using a PTC ceramic as a heating element is shown in U.S. Pat. No. 4,141,327. In the fuel evaporator of this patent, a PTC ceramic body provided with a large number of open holes is disposed within an air-fuel passage in order to heat the whole of the air-fuel mixture passing therethrough.
One problem of the fuel evaporator of this patent so that the flowing resistance of the air-fuel mixture is increased by disposing the PTC ceramic body within the air-fuel passage. Another problem is that since the PTC ceramic body is cooled by the air passing through the air-fuel passage, the temperature of the PTC ceramic body does not rise rapidly.
Therefore, a large amount of electric power is required to make the temperature of the PTC ceramic body rise continuously.
Accordingly, one object of the present invention is to provide a fuel evaporator provided with a heating element made of PTC ceramic, which does not increase the flowing resistance of an air-fuel mixture passing through an air-fuel passage and which is not cooled by the air-fuel mixture.
Another object of the present invention is to provide a fuel evaporator of which temperature rapidly rises by a small amount of electric power.
Still another object of the present invention is to provide a fuel evaporator which effectively evaporates unvaporized fuel which flows along the wall of the air-fuel passage as a fuel film flow.
Further object of the present invention is to provide a fuel evaporator which is easily installed in the air-fuel passage.